Where is the thin metal expansion joint "thin"? — — First distinguish between thin wall thickness and thin structure, and don't be biased by the name
Two days ago, I met a customer. He came up and asked, "Is your thin metal expansion joint and corrugated plate particularly thin?" I asked him back, "Do you want thin wall thickness or thin length?" He was stunned for a moment. This confusion is all too common. "Thin profile" mainly refers to compact structure, short axial length, small space occupation, and displacement absorption with fewer ripples or fewer wave pitches. Thin wall thickness is another matter, which directly affects the pressure resistance and life. You reduce the wall thickness in order to pursue "thinness", and the pressure will bulge as soon as it comes up, which can't be calculated.
To put it bluntly, thin metal expansion joints save space by structural design, not weight by thinning materials. For example, if the axial displacement of 50mm is also absorbed, the general-purpose corrugated expansion joint may have to be 300mm long. The thin type can be pressed to 200mm by optimizing the wave pitch and end structure, but the corrugated wall thickness may still be 1.5mm or even 2mm. In order to grab the project, many manufacturers deliberately report the wall thickness to be very thin. It looks like the structure is compact, but in fact, they exchange safety for size. Do you dare to use this "thin"?
How to choose when space is limited? Look at the three indicators of displacement, pressure and temperature together, don't focus on the thickness alone
When choosing a thin metal expansion joint, the most taboo thing is to ask "how thin can the thinnest be". What you should ask is: "In this installation space, can the three parameters of displacement, pressure and temperature be satisfied at the same time?"
Displacement determines the number of corrugations and wave pitch, pressure determines the wall thickness and reinforcing ring, and temperature determines the material. The three of them pull one and move the whole body. For example: a steam pipe, the pressure is 1.6MPa, the temperature is 350℃, the axial displacement is 40mm, and the installation space is only 200mm. You forced a single-layer thin-walled expansion joint, the number of ripples was not enough, the displacement was fully pressed on a wave, and it cracked in a few cycles. What about that? The methods are: use multi-layer thin-walled structure instead, or use external pressure single axial expansion joint, so that the working state of corrugation is more reasonable, but the length of external pressure type will increase. Therefore, when the space is limited, the three parameters must be reported to the manufacturer, so that the technical party can do fatigue check, instead of patting their own head.
Let's talk about the temperature. It is also thin, and stainless steel is fine at room temperature. When it reaches above 600℃, the creep strength of ordinary austenitic stainless steel drops linearly. You still care about "thin or not" at this time? Choose temperature-resistant alloys first. Remember when selecting: displacement, pressure and temperature are an intersection, and thinness is only the result, not the goal.
Installation pre-displacement and tie rod adjustment: the most prone to stumbling of thin expansion joints
Thin metal expansion joints have a low tolerance for installation errors because of their short length. Pre-displacement is intentionally pressing or elongating the expansion joint in the cold state, so that it is just in the middle position in the hot state. If this thing is not done properly, it will be wasted.
And guess what? Many on-site installers don't care about the pre-displacement on the drawings at all, and they come up hard. As a result, as soon as the pipeline heats up, the expansion joint is directly pushed to death, and the corrugation is pulled to the limit, which causes plastic deformation at one time. Especially for the thin structure, the number of corrugations is small, the displacement distribution of each wave is more tight, the pre-displacement difference is 5mm, and the corrugation life may be directly cut in half.
The same goes for tie rod adjustments. The tie rod is not for you to screw casually, it is used to distribute displacement and limit internal pressure thrust. For example, there are clear data on how much loose and tight the tie rod nut is for the transverse expansion joint of the compound hinge. The correct method is: first loosen the locking nut, adjust the middle nut to the specified opening, and then lock it tightly. But what about the scene? Often, it is twisted with a big wrench until it can't be moved-the tie rod becomes a rigid support, and the expansion joint becomes a dead pimple. How can we compensate for the displacement?
Failure mostly starts with corrugation: fatigue life, guide tube and stiffness. These data should be read
After-sales feedback problems of thin metal expansion joints, nine times out of ten, the corrugation is broken. Either fatigue cracks or erosion perforations. You have to ask the manufacturer for three numbers: fatigue life, guide tube configuration and stiffness.
Fatigue life does not say "can last for ten years", but refers to the number of cycles under specified displacement, pressure and temperature. For example, the design number of cycles of a thin expansion joint is 10,000 times, but according to the actual working conditions, you have to calculate how many times to start and stop every day and how many cycles a year. A power plant system, with two hot and cold cycles a day, 730 times a year, has a design life of 10,000 times, which is more than 13 years in theory. But if you choose the displacement larger, the actual displacement is only half of the design value, and the life will increase exponentially; In turn, when the actual displacement is exceeded, the life span will drop.
Deflectors are more critical, especially pipes with particles or high-velocity fluids. Without the guide tube, the medium directly washes the inner wall of the corrugation, and the thin-walled corrugation can be worn out in a few months. With the guide tube installed, the medium goes along the tube, and the corrugation is only responsible for deformation and does not touch the medium. Many "thin types" save the guide tube in order to save space-what is saved is protection, not cost.
The stiffness directly determines the force on the pipe support. The stiffness of the thin expansion joint is generally smaller than that of the general type, because the corrugation pitch is small and the material is thin. However, there are problems with too little stiffness. The pipe is prone to instability, and the bracket may not be able to withstand it. So when you get the parameter table, not only look at the displacement and pressure, but also the spring stiffness value, calculate the pipeline thrust, and don't let the bracket become a "bracket".
In the final analysis, thin metal expansion joint is a systematic project, and compact structure is an advantage, but every parameter has to be clearly picked out. Thin wall thickness, short length and long life-it is impossible to have both. Which one do you give priority to? Be aware of it.